The present invention relates generally to resilient elastomeric mounts with improved durability and, more particularly, to a natural rubber engine mount which is capable of long term performance at temperatures in excess of 250xc2x0 F.
Internal combustion engines and other types of vehicle power plants produce various forms of vibrations which may be undesirably transmitted to the vehicle frame. These vibrations may be high or low in amplitude and high or low in frequency. Elastomeric materials and in particular natural rubber, have been used in underhood applications. Natural rubber is a preferred elastomeric material in such applications due to its excellent mechanical properties and flex fatigue life. However, increases in underhood temperatures due to small crowded engine compartments, have led to increased temperature standards for underhood elastomeric materials. For example, whereas resilient elastomeric parts such as engine mounts and bushings were previously required to meet a temperature requirement of 200xc2x0 F., the same parts must meet temperature requirements of 250xc2x0 F. to 300xc2x0 F. Unfortunately, conventional natural rubber used in natural rubber mounts approaches its limit of resistance to oxidative degradation at temperatures over 200xc2x0 F.
The oxidation of rubber is a complicated process, involving several reactions, each of which is influenced differently by conditions. The actual degradation process occurrs during the service life of rubber products can be separated into thermal, oxidative and photochemical factors. Thermal breakdown of natural rubber involves cleavage of long chain molecules resulting in the formation of very reactive radicals. The degradation reactions are chain mechanisms in which the activated intermediates are radicals. The changes in physical properties are the sum of simultaneous processes of crosslinking, cyclization, continued polymerization and scission.
It would thus be desirable to provide resilient elastomeric mounts which are durable at temperatures in excess of 250xc2x0 F. It would also be desirable to provide resilient elastomeric mounts such as engine mounts which are made of natural rubber and are durable at temperatures in excess of 250xc2x0 F.
Resilient elastomeric mounts and, in particular, natural rubber engine mounts, which maintain good mechanical and dynamic properties at temperatures in excess of 250xc2x0 F., are provided. The natural rubber engine mounts of the present invention are thus ideal for use in underhood applications which require durability at high temperatures, i.e. from about 250xc2x0 F. to about 280xc2x0 F.
The resilient elastomeric mounts of the present invention comprise natural rubber produced by an xe2x80x9cefficient vulcanizationxe2x80x9d (EV) system. Sulfur vulcanization is the most widely used method for cross linking natural rubber. The amount of sulfur used in the natural rubber compounds varies from about 3.5 phr to about 0.4 phr. When the amount of the sulfur is reduced, an optimum crosslink density is maintained by increasing the accelerator concentration. In an EV system, the natural rubber contains a high accelerator:sulfur ratio. In general, the accelerator:sulfur ratio is about 2.5 phr to 1.2 phr. The high accelerator:sulfur ratio leads to a natural rubber containing a majority of monosulfidic crosslinks which are more heat stable than polysulfidic crosslinks found in conventional rubber. In addition, with conventional rubber, the main-chain modification and potential for oxidation is greater, even in the presence of antioxidants.
The natural EV rubber used in the mounts of the present invention comprise a curing system having a sulfur donor and accelerator, wherein the amount of sulfur donor is about 0.1 phr to about 0.4 phr and the amount of accelerator is about 1.0 phr to about 1.8 phr. The natural EV rubber of the present invention further comprises antidegredants such as antioxidants. In addition, additives such as fillers may be used to meet dynamic requirements. The engine mounts of the present invention may be manufactured using the natural EV rubber described herein, using methods known to those skilled in the art.
Those skilled in the art will appreciate that the exact compositon of the natural EV rubber of the present invention may be varied according to the desired characteristics of the resulting rubber. Moreover, it will be appreciated by those skilled in the art that various compounding changes and modifications may be made to meet various dynamic requirements for the resulting mounts, including but not limited to, static or dynamic spring rate, dampening coefficient and loss tangent. Furthermore, it will be appreciated that the present invention is not limited to resilient elastomeric mounts, but includes vibration isolation apparatus generally, made from the natural EV rubber described herein.
Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims.